A light-emitting diode (LED) is an important type of solid-state devices that convert electric current to light. It comprises an active layer of semiconductor material located between two oppositely doped layers, one being p-type and the other being n-type. A drive voltage is applied across electrical contacts on the doped layers causing electrons and holes to be injected from the doped layers into the active layer. The electrons and holes then recombine to generate light that emits omni-directionally from the active layer and escapes from all the surfaces of the LED. Generally, the active layer is a single-quantum-well (SQW) layer or multi-quantum-well (MQW) layer. The MQW active layer is expected to achieve an improved device characteristic such as higher output as compared to an SQW active layer because the MQW active layer can emit light efficiently at a small current due to a plurality of mini-band structure.
FIG. 1 shows a conventional LED device having an MQW active layer with good efficiency and output. As shown in FIG. 1, the light-emitting device 10 comprises an MQW active layer 15 between a p-type nitride semiconductor layer 14 and an n-type nitride semiconductor layer 13. All the barrier layers, except the barrier layer next to the p-type nitride semiconductor layer 14, of the MQW active layer 15 are doped with n-type dopants. Moreover, there is another conventional LED device 20 as shown in FIG. 2. The light-emitting device 20 comprises an MQW active layer 25 between a p-type nitride semiconductor layer 24 and an n-type nitride semiconductor layer 23. Only several barrier layers near the n-type nitride semiconductor layer 23 of the MQW active layer 25 are doped with n-type dopants.